Research On Evolution Law And Control Method Of Void Defects Of Large Ingot In Upsetting Process

Large machinery and heavy equipment are important embodiment of a country's ultimate manufacturing capacity,and also an important embodiment of a country's scientific and technological level and comprehensive national strength.These large and heavy equipment are widely used in aviation,aerospace,navigation,energy,electric power,chemical industry,mining and other fields,with a pivotal strategic position.Large ingots are often used in the manufacturing process of large and heavy equipment.However,due to liquid shrinkage,solidification shrinkage and solid shrinkage in the solidification process of metal materials,void defects such as shrinkage cavity and porosity often occur in the final solidification zone of ingots.When such defects fail to close in the process of thermoplastic processing,they will remain in the parts,destroy the continuity of the material,and develop into crack sources under working load,leading to fracture failure of the parts.In order to explore the evolution law of void defects in the upsetting process of large ingot,formulate the control method of void defects,and promote the closure of void defects,the following aspects were studied in this paper:(1)Research on the factors influencing the evolution of void during upsetting of large ingots.In order to find out the main factors that affect the evolution process of void defects in large ingot during upsetting,the upsetting process of 8Cr5 Mo V ?850ingot was selected as the research example.The effects of initial temperature,pressing velocity,friction condition,billet shape,billet size,void shape,void size and void position on void evolution process were studied.Through the research on the evolution process of the void under different conditions in the upsetting process,the evolution law of void is found out,the main factors affecting the evolution of void are obtained,and it was concluded that the stress-strain state of the void location is the fundamental factor affecting the void closure.(2)Research on void closure law based on equal stress and strain model.In order to study the relationship between the tress-strain state and the evolution process of voids,an equal stress and strain finite element model(ESS model)was proposed.With the help of this model,the evolution law of spherical void under uniaxial stress state,biaxial stress state and triaxial stress state was studied.It was found that there are four main forms of void evolution under different stress states.It was determined that the stress axis is the fundamental factor affecting the evolution process of the void.Under the stress state with the same stress axis,the evolution law of the void is the same,and the maximum compressive strain component(absolute value)required for void closure is also the same.And the critical closure criterion of the void under arbitrary stress state in the whole three-dimensional stress space was established.(3)Research on the formation reason and control method of the void difficult to close area in large ingot during upsetting process.In order to solve the problem,combined with the proposed three-dimensional stress space void closure criterion,the reasons for the formation of the void difficult to close zone at the end of the billet were explored,and the factors affecting the size of the void difficult to close zone were explored.Finally,through the optimization of anvil surface,the void difficult to close zone at the end of the billet was well controlled.